1. Technical Field of the Invention
The present invention relates generally to a gas sensor control system which is designed to sample an output from a gas sensor such as a NOx sensor to determine the concentration of gas, as measured by the gas sensor, accurately.
2. Background Art
Recently, automotive emission regulations and fuel efficiency requirements have been more tightened. For instance, there are increasing needs for techniques for reducing the amount of NOx emissions from diesel engines or diagnosing a NOx purifier. Modern gasoline engines are controlled to burn fuel over a fuel-lean range in order to improve the fuel efficiency, thus resulting in an increase in amount of NOx emissions. Techniques for controlling the NOx emissions or diagnosing the NOx purifier are, therefore, been required. For these reasons, NOx sensors are increasingly demanded.
The NOx sensors are typically of a multi-cell structure using a zirconia made solid electrolyte body. For example, the NOx sensors are equipped with a pump cell, a monitor cell, and a sensor cell which are made of solid electrolyte. The pump cell works to keep the concentration of oxygen (O2), as contained in exhaust gas introduced into a gas chamber of the NOx sensor, at a constant value. The monitor cell works to measure the concentration of NOx contained in the exhaust gas from which the oxygen has been removed by the pump cell. Similarly, the sensor cell works to measure the concentration of NOx contained in the exhaust gas from which the oxygen has been removed by the pump cell.
Japanese Patent First Publication No. 2004-108788 teaches a gas sensor control system using the above type of NOx sensor. The gas sensor control system is designed to measure an electric current flowing through the sensor cell (which will also be referred to below as a sensor cell current Is) and an electric current flowing through the monitor cell (which will also be referred to below as a monitor cell current Im) using measuring circuits, calculate a difference between the sensor cell current Is and the monitor cell current Im (i.e., Is−Im), and determine the concentration of NOx as a function of such a difference. This enables the concentration of NOx to be determined regardless of the concentration of oxygen contained in gas introduced into the gas chamber of the NOx sensor.
The sensor cell current Is, as produced by the sensor cell of the above type of NOx sensor, usually includes a current component arising from to the concentration of NOx to be measured and a current component arising from the concentration of oxygen remaining in the gas chamber of the NOx sensor, thus causing the resolution of the measuring circuit in measuring the sensor cell current Is to be decreased by an amount corresponding to the concentration of oxygen remaining in the gas chamber. The sensor cell current Is that is a function of the concentration of NOx is very faint in level of the order of nA, which results in an increase in ratio of the current component arising from the concentration of oxygen to that arising from the concentration of NOx. There is left a room for improvement of accuracy in measuring the concentration of NOx.